There are several companies that make manual "all terrain wheelchairs". These chairs are essentially regular wheelchairs with four larger tires. Are they worth it? I would say no. They are expensive and too heavy. You could spend 75% less by making temporary modifications to your everyday wheelchair. If you buy a second set of large wheels with big tires and a Freewheel Wheelchair Attachment you will get the same results. Plus, you don't have an extra piece of gear to lug around. If there isn't any mechanical (gears) or power assist advantage they are not worth the extra cost.